This invention relates to subscriber loop interface circuits and, more specifically, the invention relates to a method and apparatus in a subscriber loop interface circuit for measuring loop impedance.
A subscriber loop interface circuit (xe2x80x9cSLICxe2x80x9d) is generally employed as an interface between a subscriber network and a subscriber terminal device (such as a residential telephone). A SLIC may provide functions such as two-to-four wire conversion, battery feed, line supervision, or common mode rejection.
Generally, a subscriber loop includes a SLIC, a subscriber terminal device (e.g., a telephone), and a pair of subscriber lines for connecting the SLIC to the subscriber terminal device. When a subscriber terminal device is in an off-hook condition, the subscriber lines, subscriber terminal device, and SLIC together may form a continuous circuit for establishing two-way communications between the SLIC and the subscriber terminal device.
Typically, a SLIC includes circuitry for obtaining information on voltage levels across the subscriber lines (loop voltage) and for obtaining information on the current flowing in the subscriber loop (loop current). Typically, the obtained information is scaled and compared to references in order to evaluate loop voltage and/or loop current.
For example, a SLIC may detect an off-hook condition using loop voltage and/or loop current. For example, if loop current and/or loop voltage reaches a predetermined threshold, a SLIC may transmit a logic signal to a SLIC controller to indicate the detection of an off-hook condition. Further by of example, information on loop current and loop voltage may be used to control the supply of power to a subscriber loop by a SLIC.
Typically, measuring loop impedance and testing subscriber loop continuity requires invasive subscriber loop access relays and expensive test equipment.
In one prior art technique for measuring subscriber loop voltage, a voltage across the subscriber line pair is measured and converted into a single narrow pulse for the purpose of indicating the magnitude of the voltage. This technique may be inaccurate due to variations in capacitance value and manufacturing tolerance and may require a high frequency clock to provide the measurement resolution needed.
Accordingly, it is an object of the present invention to provide a novel method and apparatus for measuring subscriber loop impedance.
It is another object of the present invention to provide a novel method and apparatus in a semiconductor SLIC circuit for internally measuring subscriber loop impedance.
It is yet another object of the present invention to provide a novel method and apparatus for testing subscriber loop continuity in a SLIC formed from semiconductor material.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.